Two-way carrier wave signal transmission system



L. K. SWART 2,386,515

SIGNAL TRANSMISSION SYSTEM TWO-WAY CARRIER WAVE 1944 2 Sheets-Sheet 1Filed Dec. 50

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70 SOURCE 0F 0. C. CON TROL CURRENT TO SOURCE 0F D.C. CONTROL CURRENTINVENTOR LKSWART 0, C. CONTROL CURRENT A TTORNE Y vOct. 9, 1945.

| K. SWA RT 2,386,515

TWO-WAY CARRIER WAVE SIGNAL TRANSMISSION SYSTEM Filed Dec. 30, 1944 2Sheets-Sheet 2 OF RECT/F/ED SPEECH TO .rouncs 0F g f nacolvmoL CURRENT'4 T0 CONTROL saunas C 0F GROUNDED A. c. 0!? ac.

. MORMALLY 514.550

OPPOS/NG co/voucmva DIRECT/0N or VAR/570R! we. ,wo wu INVENTOIR I ByLKSWART ATTORNEY Patented Oct. 9,1945

UNITED STATES PAT ENT OFFICE."

SIGNAL TRANSMISSION SYSTEM Leland K. Swart, Mountain Lakes, N. J.,assignor I to Bell Telephone Laboratories, Incorporated,

New York, N. Y., av corp oration of New York Application December 30,1944, Serial No. 570,591

' r 11 Claims.

' distortion due to'the switching operations required to change thedirection of signal transmission.

Another object is to effectively silence the signal receiver during theoperation of the signal transmitter at a subscribers station of atwo-way carrier telephone system. I

Other objects are to switch the frequency of an oscillator from onevalue to another, to enable an inoperative oscillator or to disable anoperative one-"under control of signaling currents in such manner as tominimize switching transients.

These objects are attained in accordance with one embodiment of theinvention in a two-way carrier telephone system employing at each of thesubscribers stations connected by a two-way transmission line, which maybe a high voltage power line, a signal receiver of the superheterodynetype and an oscillator circuit adapted for producing carrier waves ofdifferent frequencies utilized respectively for transmitting outgoingtelephone signals over the line, and for aiding in the detection ofincoming telephone signals from i the signal-modulated carrier wavesreceived over the'line.

Each station is normally conditioned for listening, and in thatcondition the oscillator circuit is arranged to supply a wave ofsuitable frequency to beat in the first detector of the superheterodynereceiver with the signal-modulated carrier received over the line, so asto produce a combination wave of the frequency to which the followingintermediate frequency receiving amplifier is tuned, and to detect theincoming-telephone signals from the amplified intermediate frequencywave and to amplify these signals in the following second detector andaudio amplifier in the usual manner. A voice operated switching deviceis responsive to outgoing telephonic signals generated at the station tochange the frequency of the carrier oscillator to that desired for thetransmitting condition, the resulting carrier frequency, which isselected to be outside the range of amplification .of the receiver so asto silence the latter during signal transmitting intervals, beingcombined in the modulator of thesignal trans- :mitter with the outgoingsignals to provide a signal-modulated carrier which is transmitted overthe line.

Among the features of the invention are various signal-controlledswitching arrangements including vacuum tubes, copper oxide varistorsorv gaseous tubes, for enabling or disabling a vacuum tube oscillator,or for changing its oscillation frequency from one value to another,without the production of objectionable switching transients.

The various objects and features of the invention will be betterunderstood from the following detailed description when read inconjunction with the accompanying drawings in which:

Fig. 1 shows schematically a subscriber's station of a two-way carriertelephone system, embodying the invention; and

Figs. 2 to 9 show schematically alternative sigrial-controlled switchingarrangements forcontrolling an oscillator which may be employed in thesystem of the invention shownin Fig. 1.

Fig. 1 shows one of a plurality of similar subscriber's stations ofa-two-way carrier telephone system, including a signal transmitter TCand a' signal receiver RC of the superheterodyne type, associated with atwo-way transmission line TL connecting these stations. The transmitterTC includes a telephone transmitter I with its associatedenergizingbattery 2, and the amplifying vacuum tubes 3 and 4 operatingrespectively as a transmitting amplifier and a plate circuittransmitting modulator. The grid circuit of the amplifier tube 3 iscoupled to the telephone transmitter I through transformer 5, and itsplate circuit is coupled through transformer 6 to theplate circuit ofmodulator tube 5. The plate circuit of the modulator tube 4 is coupledby output transformer l to the two-way transmission line TL leading toother subscribers stations. A single tube, for example, an RCA 6F8otube, may be substituted for the separate amplifier and modulator tubes3 and 4 shown.

The receiver RC is of conventional superheterodyne or double detectiontype including a first vacuum tube detector stage D1 having its gridcircuit connected through the resistance-condenser coupling circuit 8across the line TI. on;

the output side of transformer 1; an intermediate frequencyamplifier'Ar, tuned to an frequency amplifier Ar; a vacuum tube audioamintermediate -y frequency C, which may be, for example, 1'15 kilo- Iits own oscillating circuit. v frequency B from OB is applied throughthe conpressed by coupling 1 the second detector stage the modulatortube 4,

fier AA'may be included in a single tube l3 as indicated; for example,an RCA 6F8c may be used.

A carrier wave of frequency A, for example, 300 kilocycles, for theoutgoing speech wave is supplied by the oscillator A comprising a singlevacuum tube having inductively coupled plate and grid circuits and acoil-condenser frequency determining circuit IS in its plate circuit, asindicated. The output of the oscillator 0A is connected through theseries condensers I6 and I! to the grid circuit of the modulator tube 4in the transmitter TC. A similar vacuum tube oscillator Os havinginductively coupled plate and grid circuits and a coil-condenserfrequency determining circuit l8 in its plate circuit, provides acarrier wave of the frequency B, for example, 475 kilocycles.

The carrier oscillators Oil and 0e are controlled to provide two-wayoperation of the station under control of the oscillator control vacuumtube 0C and the amplifier-rectifier vacuum tube AR, which may be an RCA75-type tube, in the following manner:

In the normal receiving condition of the station, when no outgoingspeechenergy is bein applied to the transmitter TC, the oscillator On isnormally oscillating at the frequency B (475 kilocycles) since its gridis not biased to prevent such operation, the only bias being receivedfrom The carrier wave of denser l9 and the condenser of coupling circuit8 in series to the control grid circuit of the first detector stage D1of the superheterodyne receiver RC, and beats in that detector with theincoming speech modulated carrier of frequency A (300 kilocycles)received over the line TL from a talking subscriber at-another station,and also imcircuit 8 on that detectors grid circuit, to provide acombination wave in the detector output of the intermediate frequency C(175 kilocycles) to which the following intermediate frequency amplifierA: is tuned. This intermediate frequency wave is amplified by theamplifier Ar, and the modulating speech frequencies are demodulated fromthe amplified wave in following audio amplifier AA and are heard in theassociated telephone receiver l2. The oscillator 'Os continues tooscillate at the frequency B until the subscriber at the station startsto talk,

and until this occurs the oscillator 0A is maintained in anon-oscillating condition by the biasing voltage drop across the large(100,000-ohm) resistor 20 in series with its cathode provided by theplate current of the oscillatorcontrol tube OC flowing through thatresistor.

When the subscriber at the station starts to talk, his speech currentgenerated in telephone transmitter l is impressed by transformer on theamplifier 3, A portion of the amplified speech energy in the output ofamplifier 3 is applied through transformer 6 to the plate circuit of andanother portion is impressed through condenser 2| and potentiometer 22on the control grid circuit of the three-electrode amplifier portion ofthe tube AR. The amplified speech waves in the plate circuit of theamplifier of tube AR. passes through the primary winding Pl oftransformer 23 and are induced D2, are amplified by the 'tively shuntedby the condensers 26 and 21, associated with these respective windingsand the respective diode plates of the tube AR, making the upperterminals of these resistors have a negative potential with respect toground, which may be adjusted to a desired value by proper adjustment ofthe potentiometer 22 in the input of the tube AR. The negative potentialof the resistor 25 associated with the secondary winding S2 oftransformer 23 is applied directly to the grid circuit of the oscillator,control tube 0C, biasing that tube to cut-off. The resultant reductionto zero of the plate current of that tube, flowing through the cathoderesistor 20 of the oscillator tube 0A, will remove the normal negativebias on the latter tube provided by the voltage drop across thatresistor, alowing On to oscillate at the frequency A (300 cycles). Atthe same time, the negative potential from the resistor 24 associatedwith the secondary winding SI of transformer 23 is applied directly tothe grid of the oscillator tube 0a to bias that tube to cut-off,preventing the latter from oscillating. Thus, the frequency of thecarrier oscillator arrangement at the station is switched from thefrequency B to the frequency A as soon as voice energy is applied to thetransmitter TC, andi in the absence of applied voice energy, the reverseprocess takes place, the carrier frequency being switched from A to B.The selected values of the condensers 26 and 21 associated with theresistors 24 and 25 will determine the speed at which the switchingtakes place, as well as the degree of hangover of the switchin circuitbetween speech syllables. The values of these condensers would be madesuch that there is sufficient holdover of frequency A so that normalpauses between syllables of a word will not result in the reversion ofthe carrier produced at the station to the frequency B, such as willoccur at the station when the subscriber thereat stops talking.

While speech energy continues to be applied to the telephone transmitterI of the transmitter TC, the oscillator 0A continues to oscillate tosupply the carrier of frequency A (300 cycles) to the grid circuit ofthe modulator 4, which carrier will be modulated in that modulator bythe speech energy applied to the plate circuit of the tube to provide aspeech modulated carrier of the frequency A (300 kilocycles) which isimpressed by transformer 1 on the line TL, and is transmitted thereoverto the other subscribers stations. When the station has been changedfrom a listening to a talking condition in the manner which has beendescribed, the frequency A (300 cycles) of the signal-modulated carrierwave apearing on the line at theinput to the receiver RC is so far awayfrom the intermediate frequency C cycles) to which the receiver of thestation is tuned that it will not be amplified in the receiver, and thetelephone receiver I2 of the station will be silenced.

When the subscribers at two connected stations both talk at once, eachwill cause the oscillator at his station to beswitched to frequency Aand ,The oscillator of eac coming signal-modulated carrier. vThus, the

- of undesirable large starting" and stopping" transients tending toproduce clicks or. other distortion in the transmitted speech; Suchswitching transients would be attained in systems in which the requiredswitching involves thejntroduction in or removal of considerable lossesfrom thesignal transmission paths, or making changes ,in the gain ofthese paths by biasing the grids of vacuum tubes negatively so as toswitch substantial amounts of energy in the anode circuits of thesetubes, especially when the system is operating at a frequency notgreatly removed from the voice transmission; The switching arrangementof the invention has certai-, other advantages, such as a selectivelytuned high gain receiver and non-critical switching circuits.

To cut down'the numberpf tubes employ d in the system of the invention,it may be desirable to user a single double triode tube in place of theseparate oscillator tubes A and OB], forexample, an RCA 6C8 tube; andalso a single double triode tube to replace the detector tube D1 in thereceiver RC and the oscillator control tube 00, for example, an RCA 6E8ctube.

..Figs. 2 to 9 show alternative arrangements which may be employed forswitching the frequency of a vacuum tube oscillator from one value toanother, or for enabling or disabling such an oscillator, withoutchanging the static currents in the vacuum tube, so as to avoid theproduction of objectionable switching transients.

of these figures comprises a single vacuum tube 0 having its platecircuit inductively or c-apacitively coupled to its grid circuit, andhaving a parallel inductance coilcondenser arrangement associated withthe coupled circuits for determining the frequency of oscillation.

In the circuit of Fig. 2 the coil of an inductance L1 in the platecircuit of the oscillator tube 0 is inductively coupled to a coil ofinductance L2 .in the grid circuit of the tube, and a third inductancecoil of inductance L3, which may or may not be inductively coupled to L1or L2, is connected in a closed series circuit with the inductance coilL2 and.the condenser C1 to form a frequency-determining or oscillatingcircuit for the oscillator tube. The varistors VRI and VR2, which arepreferably of the copper oxide rectifier type, are connected in seriesacross the inductance coil La, and arerelatively poled in such adirection that they in effect constitute an open circuit across thatcoil. A source of direct current, which may befrectified speech; isadapted to be connected by the switch S across the midpoint of the coilL3 and a point between the varistors VRI and V'R2. With the switch Sopen, the frequency of oscillation of the oscillator is determined bythe selected values of the inductances L2 and In in series and the shuntcapacity C1. When the switch S is closed to apply a direct currentbiasing potential poled as indicated across the inductance Le, theimpedance characteristics of the varistors VRI and VR2 will be sochanged that they effectively placea low resistance shunt across thatinductance, and

since this materially reduces the inductance of the oscillating circuit,th period of the oscilla coils L1 and L2. In the circuit of Fig. 4 thecapacity C1 is also connected in shunt with the inductance L2 in thegrid circuit of the oscillator tube, but the inductance L3 is connectedin series with the normal high impedance varistors VB! and VR2 acrossthe coil L2 and shunt capacity C1, so that the inductance L2 iseifectively removed from the oscillating circuit in the normal conditionof that circuit; and the source of direct control current is adapted tobe connected by the closing of the switch S directly across the varistorVRI, and across the 'varistor VR2 in series with theradio frequency coilR. F. C. provided to prevent shunting of oscillations from L1 and L2.The closing of the switch S to connect the source of direct controlcurrent across the ivaristors in the circuit of Fig. 4 will change theimpedance of the varistors VB! and VR2 so oscillating circuit and thusincrease the period of oscillation of the oscillator.

- Figs. 5 b0 show various arrangements employing cold cathode gas tubesfor changing the operation characteristics of a single vacuum tubeosci1lator.- In the arrangement of Fig. 5, with the switch S opened, theoscillator will generate a wave of a frequency determined by theinductance L1 in series with the inductance le and the shunt capacity C!in the oscillation circuit connected in series with the condenser C2 inthe feedback path between the plate and the control grid circuit of theoscillator tube 0. The cold cathode gas tubes GTI and GT2, shunted re-'spectively by the resistors R1 and R2, are connected eifectively inseries with each other across this oscillating circuit, and are normallydeionized so as not to interfere with the oscillation of the oscillator.When the switch S is closed to apply a direct current voltage from theassociated direct current source, which may be a rectified speechsource, as indicated, across the mid-point between the coils L1 and L2and the mid-point between tubes GTI and GT2, of suificient value, say,70 volts, to cause ionization in these tubes, a relatively lowresistance shunt will be placed on the oscillating circuit effectivelydisabling the oscillator.

In the circuit arrangement of Fig. 6 the vacuum tube oscillator Onormally oscillates at a period determined by the inductance L1 inseries with the inductance L2 and the shunt" capacitance C1 in theoscillation circuit connected in series the gas in that tube to providea relatively low resistance shunt through'that tube and the ca-- pacityC3 across the inductance L1. Thi will reduce the total inductance of theoscillating circuit and increase the period of oscillation of theoscillator.

In the circuit of Fig. '7 the plate of the oscillator tube is connectedto the grid thereof through the series circuit comprising the condenserC2 and a circuit comprising the capacity C1 shunted by the inductancesL1 and L2 and the intermediate cold cathode gas tube GT4. With theswitch S open, the gas tube GT4 i deionized so that it effectivelyprovides an open circuit between the inductances L1 and la in theoscillating circuit and therefore the oscillator normally will notoscillate. When the switch S is closed, a source of direct currentvoltage, which may be rectified speech, of sufficient value, say, '70volts, to cause ionization in the tube GT4 is applied across that tube,the resulting ionization in that tube causing the connection between theinductances Ll and L2 to be effectively closed. This will cause theoscillator to oscillate at a period determined by the values of theseries inductances L1. and Lo and the shunt capacity C1.

The oscillator circuit of Fig. 8 normally oscillates at a perioddetermined by the values of the inductance L1 in series with theinductance L2 and the shunt capacity C1 in the frequency-determiningcircuit connected between the plate and the grid of the oscillator tube.The copperoxide varistors VR3 and VR4 are connected in series across theinductance L1 in this oscillating circuit andare so poled that theyprovide a very high impedance path in shunt with that inductance andthus do not normally prevent oscillation of the oscillator. When theswitch S is closed to connect a direct current source of biasingpotential poled as indicated, through the series resistance R3 and shuntcapacity C3 across a point between the two varistors VR3 and VR4 and themid-point of the inductance coil L1, these varistors will become a lowimpedance and will effectively short out the inductance coil L1, thusdecreasing the inductance in the oscillating circuit and causing theoscillator to oscillate at a. higher frequency. The radio frequency coilR. F. C. in series with the biasing circuit is provided topreventshunting out or suppressing oscillation by virtue of theconnection of C5; or the direct current control circuit to the mid-pointof L1.

In the oscillator circuit of Fig. 9, the two copper-oxide varistors VH3and VR4 are connected in shunt with the oscillating circuit comprisingthe inductances L1 and Le shunted by the capacity C1, connected inseries with condenser C2 between the plate and grid of the oscillator 2,instead of in shunt with the inductance L1 alone as in the circuit ofFig. 8, and a source of control voltage, which may be a groundedalternating current or a direct current source poled as shown is adaptedto be connected by switch S across the mid-point between the twovaristors and the mid-point between the inductances L1 and In. Therectifying arrangement comprising the series copper-oxide varistor VR5and the shunt capacity C4 in the biasing circuit makes it possible toimpress a direct current bias on the varistor when either an alternatingcurrent or direct current potential is applied at the input of thecontrol circuit. Let us consider that an alternating current potentialis impressed on this control circuit on the positive half of the cycle.The varistor VR5 will pass current and charge the condenser G4 at apositive potential with respect to ground. This positive potential willmaterially reduce the impedance of the varistors VR3 and VR4 thuseffectively short-circuiting the oscillating circuit and stopping theoperation of the oscillator. This direct current bias will also bepresent during each. negative half cycle of the impressed voltagebecause of the energy fed from the condenser C4 during this period.

The rectifying arrangement of Fig. 9 may be applied also to any of theoscillator circuits of Figs. 2 to 8 to provide a direct current sourceof control potential from an alternating current source.

The variable frequency oscillator of Figs. 2, 3, 4, 6 or 8 maybesubstituted for the oscillator arrangement comprising the oscillatortubes 0A and 0a, the control tube 00 and the associated grid biasingarrangements in the system of Fig.1, in which case the direct currentbiasing potential for switching the frequency of the former oscillatorfrom one value to another would be obtained from the output of theamplifier-rectifier AR during the period in which speech energy isapplied to the transmitter TC. Also, the normally operative oscillatorand associated disabling arrangement of Fig. 5 or Fig. 9 may besubstituted for the normally operative oscillator 0A and associateddisabling arrangement in the system of Fig. l, and the normally disabledoscillator and associated enabling arrangement of Fig. 7 may besubstituted for the normally disabled oscillator On, control tube OC andthe associated oscillator enabling arrangement in the system of Fig. l,the direct current control for the substituted arrangement in each casebeing supplied from the output of the amplifier-rectifier AR.

Various modifications of the circuits illustrated and described whichare within the spirit and scope of the invention will occur to personsskilled in the art.

What is claimed is:

1. In combination at a station of a two-way carrier wave signalingsystem, a signal transmitter including a signal modulator, a signalreceiver including a signal demodulator, a common oscillator forsupplying carrier waves of difswitching device responsive to outgoingsignals enerated at the station to change the produced frequency of saidoscillator to that used for transmitting said outgoing signals.

2. The combination of claim 1, in which said oscillator comprises oneportion normally in oscillating condition to supply to said demodulatorthe carrier wave of the frequency used during said receiving intervals,and another normallydisabled oscillator portion, which, when operative,oscillates to supply to said modulator the carrier wave of the frequencyused for transmitting outgoing signals, said signal-operated switchingdevice being responsive to the-outgoing signals generated at the stationto disable the first oscillator portion and to enable said otheroscillator portion during signal transmitting intervals.

3. The combination of claim 1, in which said common oscillator comprisestwo vacuum tube oscillator portions each having a control grid, thegrids of said oscillator portions being noranally biased so that oneportion is in a state of oscillation to supply a carrier of the desiredirequency to said demodulator, and the other portion is disabled, saidsignal-operated switching device being responsive to outgoing signalsgenerated in said transmitter to reverse the bias on the grids of saidoscillator portions 50 that said one portion is disabled, and said otheroscillator portion is enabled to supply to said modulator a carrier waveof the frequency used for transmitting said outgoing signals.

4. In combination in a two-way carrier wave telephone system, a two-waytransmission line and a subscribers station comprising a signaltransmitter including a wave modulator feeding said line, a signalreceiver of the superheterodyne type, including a wave detector fed fromsaid line and a following wave amplifier tuned to a given intermediatefrequency, a carrier wave source comprising one oscillator portionnormally operative to transmit a carrier wave of suitable frequency tosaid detector to beat therein with an incoming signal-modulated carrierwave of another frequency received by said receiver fromv said line, soas to produce a, combination wave of said intermediate frequency, and asecond normally disabled oscillator portion, which, when operative,transmits to said modulator a carrier wave of a third frequency tocombine therein with outgoing voice signals generated in saidtransmitter to produce a signal-modulated carrier wave for transmissionover said line, a voiceoperated switching device responsive to voicesignals generated in said transmitter to disable said one oscillatorportion and enable said second oscillator portion andresponsive tocessation in the generation of said signals to return said station tothe normal condition with said one oscillator portion operative and saidsecond oscillator portion disabled, said third carrier frequency beingoutside the amplification range of said tuned intermediate frequencyamplifier. I 5. The combination of claim 4. in which said VQice operatedswitching device comprises means connected to said transmitter foramplifying and detecting a portion of the voice signals generatedtherein, and means responsive to the detected signals'to produce biasingvoltages providing the desired reversal of the operating condition ofsaid two oscillator portions.

6. The combination of claim 4, in which each of said oscillator portionscomprises a vacuum' tube oscillator having a control grid, the normaldisabling of said second oscillator portion is provided by a controlvacuum tube having a control grid, normally operative to bias thatoscillator portion against operation, said voice-operated switchingdevice comprises an amplifier-rectifier supplied with a portion of thevoice signals generated in said transmitter, and means responsive to therectified voice output of said amplifierrectifier to apply a disablingbias to the grid of said one oscillator portion, and a disabling bias tothe grid of said control tube so as to effectively remove the disablingbias produced thereby on said second oscillator portion.

7. In combination, a vacuum tube oscillator having two difierentfrequency determining circuit arrangements, means normally making onlyone of said frequency-determining arrangements effective so that saidoscillator normally produces a wave output at one frequency, a source ofdirect current, and means responsive to the application of waves fromsaid source to said oscillator to cause said other frequency-determiningcircuit arrangement only to be eflective so that said oscillatorproduces a wave output of a different frequency.

8. In combination, a vacuum tube oscillator having afrequency-determining circuit including inductance and capacitance,variable impedance means connected across at least a portion of saidinductance, the shunt impedance provided by said variable impedancemeans being normally so high as to constitute an effective open circuitacross said inductance portion making the latter effective in saidfrequency-determining circuit to cause oscillation of said oscillator ata frequency determined by the total inductance and capitance in saidcircuit, a direct current source, and means responsive to the currentfrom said source to change the shunting impedance of said variableimpedance means to a very low value, thereby effectively removing saidinductance portion from said frequency-determining circuit and causingsaid oscillator to oscillate at a dillerent frequency.

9. The combination of claim 8, in which said variable impedance meansacross said inductance portion comprises two copper oxide varistorspoled in oposition providing such a high shunt impedance that saidinductance portion is effective in said frequency determining circuit,and said source applies a direct current bias to said varistor of suchvalue and with such poling as to reduce the impedance thereof to such alow value as to effectively short-circuit said inductance portion andthus increase the frequency of oscillation of said oscillator.

10. The combination of claim 8, in which said variable impedance meanscomprises one or more gaseous discharge devices which are normallydeionized to provide a very high shunt impedance across said inductanceportion so that the latter is effective in said circuit, and are ionizedin response to the direct current bias applied by said source toeffectively short-circuit said inductance portion and thus change thefrequency of oscillation of said oscillator.

11. The combination of claim 8, in which said variable impedance devicecomprises tWo oppositely poled copper oxide rectifiers shunting thewhole inductance in said frequency-determining circuit, said rectifiersnormally providing an ef- "fective open circuit across said Wholeinductance and thereby allowing oscillation of said oscillator at afrequency determined by the values of said inductance and capacitance,and being biased by the current from said source to reduce the impedanceof said rectiflers to a very low value so as to effectivelyshort-circuit said whole inductance and thus to stop oscillation of saidoscillator.

LELAND K. SWART.

